Oklahoma will use a new and untried combination of compounded drugs in the executions of two men, according to an email the state sent to attorneys on Tuesday.

Assistant attorney general John Hadden said the state plans to employ a lethal three-drug combination of midazolam, pancuronium bromide and potassium chloride. The combination is believed never to have been tried before in US executions.

The disclosure is the latest development in an ongoing series of legal challenges in several states, as lawyers for death row inmates seek to challenge the methods proposed for their executions, and the secrecy surrounding the sources of drugs. States, meanwhile, are battling shortages in the supply of drugs, after an EU-led boycott severely limited their options.

In Texas, lawyers for two murderers on death row filed a lawsuit in a Houston federal court on Tuesday attempting to stop the inmates' imminent executions on the basis that the state department of criminal justice is refusing to reveal details about the compounded pentobarbital to be used in the two lethal injections.

In Oklahoma, Hadden declined to reveal the source of the drugs to lawyers. He said the corrections department has purchased the drugs, and an unidentified pharmacy is holding the drugs until within 24 hours of the executions. The midazolam and pancuronium bromide is compounded, he said, and the state has ordered an analysis of the drugs. The results would be provided to the defense and redacted to conceal the identity of participants. The potassium chloride was manufactured, and the state will call for no "special testing" on it, he said.

Defense attorneys have raised questions about the efficacy and purity of compounded drugs which lack federal approval.

"The drugs to be used are midazolam, pancuronium bromide and potassium chloride in the dosages indicated in the protocol," Hadden wrote. "Also, DOC can make additional disclosures on compounded drugs (which in this case are the midazolam and pancuronium bromide) to answer questions regarding purity, strength and competence of the pharmacy/pharmacist."

An Oklahoma county district court judge ruled last week that a state law keeping the source of the drugs a secret is unconstitutional and denies the men their right to access the courts to argue against their own executions.

Oklahoma plans to execute Clayton Lockett on 22 April and Charles Warner on 29 April. Both have been convicted of rape and murder. Hadden acknowledged in his email to lawyers that he was sharing the information about the drugs to be used because "time is a factor".

Richard Dieter, executive director of the Death Penalty Information Center, said Oklahoma's planned three-drug method is similar to what Florida has used in its executions. Florida has used midazolam, vecuronium bromide and potassium chloride.

Florida and Ohio are the only states to use midazolam in lethal injection. The January execution of Dennis McGuire using midazolam and hydromorphone took longer than 20 minutes.

Jen Moreno, staff attorney at the Berkeley Law Death Penalty Clinic, said Florida's protocol using vecuronium bromide and Oklahoma's using pancuronium bromide are essentially interchangeable. "It will function no different than the paralytic Florida is using," Moreno said.

A main difference in the protocols is the dosage of midazolam. The dosage in Oklahoma's protocol is much lower than Florida's, Moreno said. The purpose of midazolam would be to cause unconsciousness so the prisoners would not feel paralysis, suffocation and pain.

"With a lower first dose, it increases the risk he will be conscious and experience the effects of the second and third drug," Moreno said. "There would be serious concerns it wouldn't operate as it's intended to operate in the protocol."

Madeline Cohen, an attorney for Warner, said it is unclear how a compounded, lower dosage of midazolam would perform. "This is a very, very troubling protocol," she said. The inmates' lawyers plan to file for a stay of execution.

Oklahoma revised its lethal injection protocol in March to allow five different ways to kill condemned inmates by lethal injection. There are four three-drug methods or a single large dose of pentobarbital. The warden can choose the method, according to the protocol.

Dieter said states have turned to compounding pharmacies as a source for execution drugs, as major drug companies have objected to states using their federally approved drugs in executions.

"The drugs are prepared from raw ingredients by a pharmacist who may not be familiar with all the secondary factors such as temperature, sterility and timing that would be used by a manufacturer in pristine conditions," Dieter said.

"Each pharmacist may prepare the drug slightly differently, and even one day's dose may differ from another. Moreover, there has been recent evidence of contaminated drugs from compounding pharmacies causing severe and deadly reactions. Any of these factors could cause the first drug in a lethal injection to be less than 100% effective, thereby exposing the inmate to the risk of observing his own slow death."

In the Texas case, a judge in Austin last week ordered state officials to disclose information about the drugs and suppliers to the attorneys for Tommy Sells and Ramiro Hernandez. However, Texas officials appealed and on Friday another court issued a stay pending a full hearing. That may not take place until mid-April at the earliest, but Sells is set to die this Thursday and Hernandez six days later.

Their lawyers are arguing that Texas law says the information should be public and that not knowing the origins of the drugs and being able to make an assessment about their quality potentially exposes their clients to an unnecessarily painful execution that would violate their constitutional rights. Maurie Levin, one of the attorneys, said that she is prepared to appeal to the US supreme court if necessary.